I would think that a better mitral valve would be less of a risk than a really badly faulty valve, go with the cardiologist advice, and or even get a second opinion.
If you are in England, NZ or Australia, having valves replaced on the national health, is only for the seriously ill, otherwise you wait, they do not waste Govt resources on the patients who not require surgery.
As I understand it even mitral valve is done now through a small incision in the groin, where they thread a folded mesh valve into your heart, and then use a balloon to inflate, done and dusted in a couple of hours, and home the next day all being well, much bigger improvement on open heart surgery.
But again listen to the cardiologists, they are the best source of information, go with a list of questions, even if they seem a little silly.
I usually have a half page of questions written out for cardio every time husband visits his cardio, he has heart failure, so a completely different beast, I have found they like you to be taking an interest, rather than just sitting there like a dummy, I have even suggested once would he not do better on a different meds, I had researched it all out, gave him other cardiologists opinion about the meds, printouts on meds, one med v's another, that I found on the internet, dropped names, and his meds were changed as per my suggestion.
WHY WHY WHY, after three years of complaints about side effects had they not considered what I came up with myself, NOT DOING THEIR JOB, or just plain LAZY.
EDUCATE YOURSELF, I cannot emphzise this enough, it has saved my husbands life and made his life a lot easier and again ask questions, I am full of questions each and every time we visit the hospital, keep a note book as they occur to me.